build-aux/generate-change-log: s/python/perl/

Replace the Python script for creating the ChangeLog file with a version
written in Perl.  The reason is simply that the Python binary on the
system I use for creating the NSCA-ng releases is called "python2.7",
not "python"; and reimplementing the thing in Perl seems simpler than
adding magic to update the Shebang line when Python is updated :-)